1. Field of the Invention
The present invention concerns an omega type zeolite having high thermal stability, its process of preparation and its utilization.
Zeolites are hydrated crystalline silicoaluminates having the following general formula: EQU M.sub.2/n O; Al.sub.2 O.sub.3 ; xSiO.sub.2 ; yH.sub.2 O
where:
M is a cation of valency n; PA0 x and y numbers higher than 1. PA0 M is a metal of valency n; and PA0 Z is an organic cation, wherein the alumina source is a crystalline aluminosilicate of which at least one part is in suspension in the mother solution. The size of the particles of the crystalline aluminosilicate in suspension is comprised between 0.5 and 100 .mu.m and preferably between 1 and 10 .mu.m.
The crystalline structure of the zeolites is comprised of AlO.sub.4 and SiO.sub.4 tetrahedra, connected between one another by oxygen atoms common to the two tetrahedra. The tetrahedra surround cage or channel shaped cavities, occupied by metallic ions and water molecules.
Through ion exchange and dehydration it is possible to set free about 50% of the total volume.
In the microporous materials thus obtained, the channels and cages have variable forms and dimensions, that determine the size of the molecules that can be adsorbed and desorbed by a given zeolite during its utilization as catalyst or for the separation of molecules.
Synthetic zeolites, which allow the adaptation of the size and the form of the cavities to well defined syntheses, or the separation of specific molecules, currently assume increasing importance.
Currently, of the 36 natural zeolites known about one third can be synthesized.
For the reactions of hydrocarbonated molecules encountered in the petrochemical industry, it is necessary to dispose of zeolites comprising pores large enough to allow the adsorption and the desorption of bulky molecules.
2. Description of the Prior Art
The synthesis of zeolites consists in crystallization of over-saturated alkaline solutions of freshly precipitated alumina and silica gels. The synthesis is performed in the presence of mineral cations (most frequently from among group I of the Periodic Table of Elements) and/or organic cations, generally of the tetraalkylammonium type {Barrer and al., J. Chem. Soc. 971, (1961)}.
U.S. Pat. No. 4,241,036 assigned to UNION CARBIDE discloses the synthesis of a large-pore zeolite through crystallization of an alkaline aluminum hydroxide solution and a silica gel in the presence of tetramethylammoniums. This zeolite called omega zeolite has a natural equivalent, called mazzite (E. Galli and al., Contrib. Mineral Petrology 1974, 45 (2) 99-105).
However, despite its structure comprising large pores, omega zeolite has not until now been successfully used on an industrial scale and the lack of interest in its use and application is due to its poor thermal stability.
The poor resistance to temperature of this zeolite is due to its proper composition. The tetraalkylammonium ions, trapped in the structure, cannot be eliminated by currently used exchange methods. Their elimination requires a heat treatment at temperatures higher than 500.degree. C., known as calcination.
When this treatment is performed in the presence of an inert gas, the decomposition of the tetraalkylammonium cations leads to a carbon deposit, thus to a drop in activity. In the presence of air or oxygen, carbon deposits can be prevented, but the exothermicity of the oxidation reaction of these cations creates hot sites in the structure. This results in the partial destruction of the network, thereby provoking a loss of porous volume, rendering fragile the whole of the structure (J. F. Cole, Advan. Chem. Ser. 1973, 583).
The limit temperature to which the zeolite can be subject in a calcination treatment under air measures its regenerating capacities during its use in an industrial process. The low thermal stability of the zeolite limits regenerating possibilities, and renders difficult, and even impossible its industrial use.